Mössbauer investigation of intermixing during ball milling of Fe0.3Cr0.7 and Fe0.5W0.5 powder mixtures.

Abstract

The intermixing of Fe and T (T=Cr,W) during ball milling of elemental powder mixtures ${\mathrm{Fe}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathit{T}}_{\mathit{x}}$, with x=0.70 for T=Cr and x=0.50 for T=W, has been followed by $^{57}\mathrm{Fe}$ M\"ossbauer spectroscopy at room temperature (RT) and by magnetization measurements for T=W. The chemical compositions have been chosen to yield final alloys or compounds which are nonmagnetic at RT to better follow the evolution of magnetic phases with milling times. For a long period of milling time ${\mathit{t}}_{\mathit{m}}$ before reaching the final stationary state, the hyperfine magnetic field distributions remain stationary in shape for both T=Cr and T=W. Only the relative weight of the magnetic contribution decreases with ${\mathit{t}}_{\mathit{m}}$. For T=W, the average moment of magnetic Fe atoms is further shown to remain constant with ${\mathit{t}}_{\mathit{m}}$. Stationary hyperfine field distribution shapes are found to be similar not only for T=Cr and W but also for T=Si (x=0.50) while published spectra suggest to add T=Al, Ti, V, Ta, Re to the latter nonexhaustive list. The stationary shape is characterized by a narrow peak located at a field close to the field of alpha iron at RT (330 kG) and by a broad, almost featureless, band from \ensuremath{\sim}50--100 kG to \ensuremath{\sim}300--320 kG. The broad band represents about 2/3 of the normalized field distribution. We deduce that the interpretation which consists in attributing the x-ray diffraction peaks of Fe-based bcc solid solutions to a single Fe-rich homogeneous solid solution must be done with care for intermediate milling times. We cannot infer from such hyperfine measurements a detailed description of the regions of the powders which are responsible for such magnetic features. We argue however that irregular interfaces between nanometer-sized Fe-rich zones and T-rich zones may play a role to explain the observed shape of the hyperfine field distributions. The general conditions (process and materials) in which such phenomena may occur remain to be clarified. \textcopyright{} 1996 The American Physical Society.